1. Field of the Invention
The present invention relates generally to a magnetic memory device and a method of manufacturing the memory device. This invention relates more particularly to a magnetic random access memory (MRAM) wherein a memory cell is formed using a magnetic tunnel junction (MTJ) element that stores information “1” or “0” on the basis of a tunnel magneto-resistive (TMR) effect.
2. Description of the Related Art
In these years, many kinds of memories that store information based on new principles have been proposed. One of them is a magnetic random access memory (MRAM) using a tunneling magneto-resistive (TMR) effect. The MRAM is disclosed, for example, in ISSCC2000 Technical Digest, p. 128, Roy Scheuerlein et al., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell.”
FIGS. 15A, 15B and 15C are cross-sectional views of a magnetic tunnel junction (MTJ) element of a prior-art magnetic memory device. The MTJ element used as a memory element of the MRAM will now be described.
As is shown in FIG. 15A, an MTJ element 31 has such a structure that an insulating layer (tunnel junction layer) 42 is interposed between two magnetic layers (ferromagnetic layers) 41 and 43. In the MRAM, the MTJ element 31 stores information “1” or “0”. The information “1” or “0” is determined, depending on whether the directions of magnetization of the two magnetic layers 41 and 43 in the MTJ element 31 are parallel or anti-parallel. The term “parallel” in this context means that the directions of magnetization of two magnetic layers 41 and 43 are the same, and “anti-parallel” means that the directions of magnetization of two magnetic layers 41 and 43 are opposite to each other.
Specifically, when the directions of magnetization of two magnetic layers 41 and 43 are parallel, as shown in FIG. 15B, the tunnel resistance of the insulating layer 42 interposed between the two magnetic layers 41 and 43 takes a minimum value. This state corresponds to, for example, “1”. On the other hand, when the directions of magnetization of two magnetic layers 41 and 43 are anti-parallel, as shown in FIG. 15C, the tunnel resistance of the insulating layer 42 interposed between the two magnetic layers 41 and 43 takes a maximum value. This state corresponds to, for example, “0”.
Normally, an anti-ferromagnetic layer 103 is provided on one of the two magnetic layers 41 and 43. The anti-ferromagnetic layer 103 is a member for fixing the direction of magnetization of one magnetic layer 41, thus permitting easy rewriting of information by merely changing the direction of magnetization of the other magnetic layer 43 alone.
FIG. 16 shows MTJ elements arranged in a matrix in a prior-art magnetic memory device. FIG. 17 shows asteroid curves in the prior-art magnetic memory device. FIG. 18 shows MTJ curves in the prior-art magnetic memory device. The principle of the write operation for the MTJ element will now be described in brief.
As is shown in FIG. 16, MTJ elements 31 are arranged at intersections between write word lines 28 and bit lines (data select lines) 32, which are arranged to cross each other. A data write operation is performed by supplying a current to each of the write word lines 28 and bit lines 32 and setting the directions of magnetization of the MTJ elements 31 in a parallel state or an anti-parallel state, making use of magnetic fields produced by the current flowing in both lines 28 and 32.
For example, in the data write mode, the bit lines 32 are supplied with only a current I1 that flows in one direction, and the write word lines 28 are supplied with a current I2 that flows in one direction or a current I3 that flows in the other direction in accordance with data to be written. When the write word line 28 is supplied with the current I2 that flows in the one direction, the direction of magnetization of the MTJ element 31 is parallel (“1” state). On the other hand, when the write word line 28 is supplied with the current I3 that flows in the other direction, the direction of magnetization of the MTJ element 31 is anti-parallel (“0” state).
How the direction of magnetization of the MTJ element 31 is changed will now be described. When a current is supplied to a selected write word line 28, a magnetic field Hx occurs in a longitudinal direction, i.e. an Easy-Axis direction, of the MTJ element 31. When a current is supplied to a selected bit line 32, a magnetic field Hy occurs in a transverse direction, i.e. a Hard-Axis direction, of the MTJ element 31. As a result, a composite magnetic field of the Easy-Axis magnetic field Hx and lard-Axis magnetic field Hy acts on the MTJ element 31 located at the intersection of the selected write word line 28 and selected bit line 32.
In a case where the magnitude of the composite magnetic field of the Easy-Axis magnetic field Hx and Hard-Axis magnetic field Hy is in an outside region (hatched region) of asteroid curves indicated by solid lines in FIG. 17, the direction of magnetization of the magnetic layer 43 can be reversed. On the other hand, when the magnitude of the composite magnetic field of the Easy-Axis magnetic field Hx and Hard-Axis magnetic field Hy is in an inside region (blank region) of the asteroid curves, the direction of magnetization of the magnetic layer 43 cannot be reversed.
In addition, as indicated by solid and broken lines in FIG. 18, the magnitude of the Easy-Axis magnetic field Hx, which is necessary for varying the resistance value of the MTJ element 31, varies depending on the magnitude of the Hard-Axis magnetic field Hy. Making use of this phenomenon, the direction of magnetization of only the MTJ element 31 located at the intersection of the selected write word line 28 and selected bit line 32, among the arrayed memory cells, is altered, and thus the resistance value of the MTJ element 31 can be varied.
A variation ratio in resistance value of the MTJ element 31 is expressed by an MR (Magneto-Resistive) ratio. For example, if the magnetic field Hx is produced in the Easy-Axis direction, the resistance value of the MTJ element 31 varies, e.g. about 17%, compared to the state before the production of magnetic field Hx. In this case, the MR ratio is 17%. The MR ratio varies depending on the properties of the magnetic layer. At present, MTJ elements with an MR ratio of about 50% have successfully been obtained.
As has been described above, the direction of magnetization of the MTJ element 31 is controlled by varying each of the magnitudes of Easy-Axis magnetic field Hx and Hard-Axis magnetic field Hy and by varying the magnitude of the composite magnetic field of the fields Hx and Hy. In this manner, a state in which the direction of magnetization of the MTJ element 31 is parallel or a state in which the direction of magnetization of the MTJ element 31 is anti-parallel is created, and information “1” or “0” is stored.
FIG. 19 is a cross-sectional view of a prior-art magnetic memory device having a transistor. FIG. 20 is a cross-sectional view of a prior-art magnetic memory device having a diode. An operation of reading out information from the MTJ element will be described below.
Data read-out is effected by supplying a current to a selected MTJ element 31 and detecting the resistance value of the MTJ element 31. The resistance value is varied by applying a magnetic field to the MTJ element 31. The varied resistance value is read out by the following method.
In the example shown in FIG. 19, a MOSFET 64 is used as a switching element for data read-out. As is shown in FIG. 19, an MTJ element 31 is connected in series to a source/drain diffusion layer 63 of the MOSFET 64 in one cell. If the gate of the MOSFET 64, which is a chosen one, is turned on, a current path is formed through the following elements in the named order: a bit line 32, MTJ element 31, a lower electrode 30, a contact 29, second wiring 28, a contact 27, first wiring 26, a contact 25, and source/drain diffusion layer 63. Thus, the resistance value of the MTJ element 31, which is connected to the turned-on MOSFET 64, can be read out.
In the example of FIG. 20, a diode 73 is used as a switching element for data read-out. As is shown in FIG. 20, an MTJ element 31 is connected in series to a diode 73 within a cell, the diode 73 comprising a P+ first diffusion layer 71 and an N− second diffusion layer 72. By adjusting a bias voltage so as to cause a current to flow to the diode 73, which is a chosen one, the resistance value of the MTJ element 31 connected to the diode 73 can be read out.
If the resistance value, which has been read out as described above, is low, it is determined that information “1” has been written. If the resistance value is high, it is determined that information “0” has been written.
In the prior-art magnetic memory device, the switching element is formed in a bulk substrate 61. In the magnetic memory device using the diode 73 as the switching device, as shown in FIG. 20, the N− second diffusion layer 72 is formed to be shallower than the bottom surface of a element isolation region 65 and the P+ first diffusion layer 71 is formed in a surface portion of the N− second diffusion layer 72, thereby ensuring electrical isolation between the present cell and adjacent cells. Hence, when the diode 73 is to be formed using the bulk substrate 61, it is necessary to form a very shallow P+ first diffusion layer 71. However, to form a shallow P+ first diffusion layer 71 is difficult because of limitations in the process, and thus it is difficult to obtain uniform diode characteristics.